Perhaps this quote from the Swiss naturalist Louis Agassiz summarizes the following pages and images best:

"I spent the summer traveling; I got halfway across my back yard."

Tuesday, September 6, 2011

A Short Primer on "Aster"ology

Summer is waning fast and early autumn is upon us, and with the season comes a last rush of bloom before the hard frosts arrive. The Aster family, formerly known as Composites, is well represented among the wildflower species. Since my knowledge of asters and goldenrods leaves something to be desired (in truth, it's virtually non-existent) it seems like a good time to take the horns by the bull and redress this situation. Two good resources toward this end are Walter Muma's Ontario Wildflowers and The ROM Field Guide to Wildflowers of Ontario.

In order to understand the species accounts it's necessary to learn to identify the various parts of the flowers. This sketch of a cross section through a typical aster below is based mostly on the diagram on Page #143 in the ROM guide (but it is my interpretation, so any errors and/or omissions are mine). Briefly, the defining characteristic of family Asteraceae is several small flowers or florets sharing a common receptacle. The florets are surrounded by rows of bracts called phyllaries, with a set of phyllaries combining to form a vase-like structure called an involucre.

The word composite usually brings to mind an image of a flower much like in my drawing: a central group of disk florets, surrounded by an outer ring of ray florets. The disk florets are composed of five petals fused into a tube-shaped corolla above the ovaries, and within the corolla are five stamens (the male flower parts) forming another ring surrounding the female style. Although very different in appearance the basic structure of the more showy ray florets is the same, except the tube of the corolla is lengthened on one side to form a strap-like ligule. You can often see the tips of the fused petals, which look like little teeth, on the ends of the ligules on flowers such as dandelions or hawkweeds.

Now let's compare the labels in the diagram above to the reality of what we see in the field. The phyllaries can be seen in these images of a Heath Aster (Symphyotrichum ericoides) ...

... a Purple-stemmed Aster (Symphyotrichum puniceum) ...

... and a Spotted Knapweed (Centaurea maculosa). In fact, the dark tips on the phyllaries are how this plant its name.

Phyllaries can be mistaken by novices such as myself for sepals, but in the aster family sepals are either absent or modified into a pappus made of bristles or hairs. And the pappus itself is attached to a small dry fruit called an achene – often mistaken for the flower's seed.

The pappus generally aids in seed dispersal and can be as little as the hooks on the achene of a beggar tick or a few tufts on the achene of a goldenrod.

Or it can form a full blown parachute such as seen on these seed heads of Yellow Goatsbeard (Tragopogon dubius) ...

... or Coltsfoot (Tussilago farfara).

A flower head made up of both disk and ray florets is called a radiate head, as in the familiar Ox-eye Daisy (Leucanthemum vulgare) ...

... and the Black-eyed Susan (Rudbeckia hirta).

In the world of nature there seem to be as many exceptions as there are rules. Flower heads can be made solely of disk florets, in which case the are called discoid heads. Some notable examples are the Pineapple Weed (Matricaria discoidea) ...

... the Tansy (Tanacetum vulgare) ...

... and the Common Burdock (Arctium minus). Most people who have spent time outdoors are acquainted with the very efficient hooks on the phyllaries, used to facilitate dispersal of the plant's fruit and hence seeds.

The flower heads of Pearly Everlasting (Anaphalis margaritacea) are also composed of disk florets, surrounded not by ray florets as one might expect, but by several rows of pearly white phyllaries.

Lastly, flowers heads can be composed solely of ray florets, in which case they are called ligulate heads, as in the ubiquitous Dandelion (Taraxacum officinale) ...

... and the Devil's Paintbrush (Hieracium aurantiacum).

That pretty much covers the basics. But many people would ask "Why bother with all this technical stuff?". The wildflower in the following three images was encountered in the forest at the Vanderwater Conservation Area. These images sat in my unidentified file for a close to a month, and the only reason I was able to eventually put a name with the images was that I stumbled across a an inset photo at the bottom of Page #165 in the ROM field guide.

With the 20/20 vision of hindsight it's obvious I should have taken more photos of different parts of the plant. In particular had I photographed (or at least had a look at!) the underside of the flower head I would have noted the presence of phyllaries. It would have been clear that the groups of tiny white flowers were disk florets (not panicles or umbels) and that this plant was a composite, leaving fewer potential candidates to sift through. A little knowledge would have made life a lot simpler ...

In fact, this wildflower is White Snakeroot (Eupatorium rugosum), a close relative of the well known Boneset (Eupatorium perfoliatum).

About a month later I found this solitary late bloomer found growing along the trail and had a chance to study this plant more thoroughly.

A side view of the flowers – clearly disk florets enclosed by phyllaries.